navigation systems, such as GPS, enable a receiver to determine a location from ranging signals received from a plurality of satellites. The ranging signals can be broadcasted on frequencies, such as the L1 signal (1.57542 gigahertz [GHz]), L2 signal (1.2276 GHz), and/or L5 signal (1.17645 GHz). L1 can have a wavelength of about 19 centimeters (cm) and L2 can have a wavelength of about 24 cm. Position can be determined from code and/or carrier phase information. A code division multiple access (CDMA) code is transmitted by the GPS satellites to the receiver and correlated with replica codes to determine ranges to different satellites, which can be used to determine the position of a GPS receiver on or near the Earth. Generally, a GPS receiver receives signals from multiple GPS satellites (e.g., four GPS satellites) to find its position.
It is desirable for receivers to be able to locate itself when no GPS signals are available or only a single such signal may be received. A wide array of techniques have been proposed to provide such position information including the use of stellar observations, inertial measurement units, signals of opportunity such as TV and cell phone signals. Each of these techniques suffers an issue of positional accuracy or lack of availability at times. For military operations, it is desirable to operate in all weather conditions, including clouds (which may obscure stellar measurements), and without reliance on TV or cellular signals in the adversaries country.
Reference will now be made to the exemplary embodiments illustrated, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended.